I'd say that the outcome of WWII which saw the US emerge as the prime world power, the availability of cheap and plentiful oil, the economic potential of a mostly unscathed country helping to rebuild a world ravaged by war and the spin-off effect from the cold war (which boosted technical development at a war-time speed in a peace-time economy) had more to do with the realisation of the American Dream than the high levels of taxation.